The purpose of this study is to evaluate the efficacy of two different doses of ddI (50mg/m2 every 12 hours or 150 mg/m2 every 12 hours) on survival, neurologic and neuropsychological tests, life threatening and opportunistic infections, growth status, immunologic tests and quantitative virologic tests. The secondary objectives are to evaluate the safety and tolerance of ddI on non-hematologic toxicity, neurologic toxicity, vital signs and other laboratory tests.